1. Field of the Invention
Embodiments of the present disclosure relate to the field of substrate processing. More particularly, the present disclosure relates to an improved method and apparatus for aligning substrates for performing successive implanting operations, such as ion implanting operations.
2. Discussion of Related Art
Ion implantation is a standard technique for introducing conductivity-altering impurities into a workpiece such as a wafer or other substrate. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the workpiece. The energetic ions in the beam penetrate into the bulk of the workpiece material and are embedded into the crystalline lattice of the workpiece material to form a region of desired conductivity.
Solar cells are one example of devices that employ silicon workpieces. Any reduced cost in the manufacture or production of high-performance solar cells or any efficiency improvement to high-performance solar cells would have a positive impact on the implementation of solar cells which, in turn, would enhance the wider availability of this clean energy technology.
Solar cells are typically manufactured using the same processes used for other semiconductor devices, and they often use silicon as the substrate material. A semiconductor solar cell is a simple device having an in-built electric field that separates the charge carriers generated through the absorption of photons in the semiconductor material. This electric-field is typically created through the formation of a p-n junction (diode) which is created by differential doping of the semiconductor material. Doping a part of the semiconductor substrate with impurities of opposite polarity forms a p-n junction that may be used as a photovoltaic device converting light into electricity.
To form solar cells, patterned doping steps are often required. Such patterned structures are typically made using traditional lithography (or hard masks) and thermal diffusion. An alternative is to use implantation in conjunction with a traditional lithographic mask, which can then be removed easily before dopant activation. Yet another alternative is to use a shadow mask or stencil mask in the implanter to define the highly doped areas for the contacts. All of these techniques utilize a fixed masking layer, either positioned directly on the substrate or in the beamline.
All of these techniques have significant drawbacks. For example, they all suffer from limitations associated with the special handling of solar wafers, such as aligning the mask with the substrate and the cross contamination with materials that are dispersed from the mask during ion implantation.
Consequently, efforts have been made to reduce the cost and effort required to dope a pattern onto a substrate. While some efforts have been successful in reducing cost and processing time, often these savings come at the price of reduced pattern accuracy. In substrate patterning processes, however, the pattern masks must be very accurately aligned, since subsequent process steps rely on this accuracy.
Thus, there is a need for a reliable, reduced cost technique for producing solar cells where the number and complexity of the patterning process steps is reduced, while maintaining adequate accuracy so that masks are correctly positioned during subsequent process steps. While primarily directed to the production of solar cells, such techniques should also be applicable to other doping applications.